1. Field of the Invention
The present invention relates to a drive device for correcting the position of an optical pickup which operates to record a signal on an optical storage medium or reads a recorded signal from an optical storage medium.
2. Description of the Prior Art
For reading a recorded signal from an optical storage medium such as a compact disc, a laser disc, or the like (hereinafter referred to as a "disc"), a laser beam is applied to the disc which is being rotated, and a light beam reflected from a mirror-finished surface cf the disc is detected by an optical pickup comprising a light detector and converted thereby into an electric signal.
The mirror-finished reflecting surface of the disc is not strictly planar, but has surface irregularities due to disc distortion. Moreover, the axis of rotation of the disc may not lie parallel to the rotational shaft of a device for rotating the disc. Therefore, the distance between the optical pickup and the reflecting surface of the disc tends to vary or fluctuate, with the result that the signal received by the optical pickup is liable to become unstable. Where the axis of rotation of the disc at the time the disc is played back is eccentrically displaced with respect to the axis of rotation of the disc at the time the signal is recorded on the disc. The reflected light beam is horizontally displaced with respect to the light detecting surface of the optical pickup, making the signal detection also unstable.
To solve the above problems, it is necessary for the optical pickup to be translatable vertically and horizontally in response to displacements of the mirror-finished reflecting surface of the disc.
Various known translating mechanisms which have been put to use will be described with reference to FIGS. 1(A) through 1(C) of the accompanying drawings.
The translating mechanism shown in FIG. 1(A) has a movable assembly 41 which includes an objective lens 1 for receiving a reflected light beam, a vertical driver coil 12 for vertically driving the objective lens 1, and a horizontal driver coil 13 for horizontally driving the objective lens 1, the movable assembly 41 being horizontally movable supported on resilient support plates 47. The translating mechanism as a whole is vertically movably connected to a base plate 42 by resilient support plates 43. The vertical driver coil 12 and the horizontal driver coil 13 are placed in an external magnetic field. The movable assembly 41 can be moved vertically by passing an electric current through the vertical driver coil 12, and can be moved horizontally by passing an electric current through the horizontal driver coil 13. The translating mechanism of this construction is disclosed in Japanese Laid-Open Utility Model Publication No. 60-155023.
In FIG. 1(B), a translating mechanism includes a movable member 41 made of a magnetic material and supporting an objective lens 1 on one end and an electromagnetic coil 45 on the other end, the movable member 41 being rotatably supported on a shaft 46. Two permanent magnets 44 are disposed in closely spaced relation to the end of the movable member 41 near the electromagnetic coil 45. The permanent magnets 44 have magnetic poles of opposite polarities confronting the movable member 41.
When the electromagnetic coil 45 is not energized, the movable member 41 is held in an intermediate position between the two permanent magnets 44. The movable member 41 can be rotated in one direction by passing an electric current through the electromagnetic coil 45 in one direction. The movable member 41 can then be rotated in the opposite direction by passing an electric current through the electromagnetic coil 45 in the opposite direction. The translating mechanism shown in FIG. 1(B) is disclosed in Japanese Laid-Open Utility Model Publication No. 58-163908.
FIG. 1(C) also shows another conventional translating mechanism. A movable assembly 41 composed of an objective lens 1, a vertical driver coil 12, and a horizontal driver coil 13 is rotatably supported on a shaft 46. The movable assembly 41 can be rotated horizontally by energizing the horizontal driver coil 13, and vertically by energizing the vertical driver coil 12. This translating mechanism is shown in Japanese Laid-Open Utility Model Publication No. 61-37127.
However, the mass moment of the movable component in each of the above conventional translating mechanisms is not in equilibrium in at least one of the vertical and horizontal directions. Therefore, the movable assembly or member 1 tends to be positionally displaced by external vibration or shocks applied to the mechanism. With the mechanisms shown in FIGS. 1(A) and 1(C), a bias voltage has to be applied to the vertical driver coil 12 in order to hold the movable assembly 41 in a vertically neutral position against gravity, resulting in increased electric power consumption.